A liquid crystal display device has been widely used as a thin, light-weight, and low power consumption display device. A liquid crystal panel included in the liquid crystal display device has a structure formed by attaching an active matrix substrate and a counter substrate together, and providing a liquid crystal layer between the two substrates. A plurality of gate lines, a plurality of data lines, and a plurality of pixel circuits each including a thin film transistor (hereinafter referred to as TFT) and a pixel electrode are formed on the active matrix substrate.
As a system for applying an electric field to the liquid crystal layer of the liquid crystal panel, a vertical electric field system and a lateral electric field system are known. In a liquid crystal panel of the vertical electric field system, an almost vertical electric field is applied to the liquid crystal layer by using the pixel electrode and a common electrode formed on the counter substrate. In a liquid crystal panel of the lateral electric field system, the common electrode is formed on the active matrix substrate together with the pixel electrode, and an almost lateral electric field is applied to the liquid crystal layer by using the pixel electrode and the common electrode. The liquid crystal panel of the lateral electric field system has an advantage of having a wider view angle than that in the liquid crystal panel of the vertical electric field system.
As the lateral electric field system, an IPS (In-Plane Switching) mode and an FFS (Fringe Field Switching) mode are known. In a liquid crystal panel of the IPS mode, the pixel electrode and the common electrode are each formed in the shape of comb teeth, and are disposed so as not to overlap each other in a plan view. In a liquid crystal panel of the FFS mode, a slit is formed either in the common electrode or the pixel electrode, and the pixel electrode and the common electrode are disposed so as to overlap each other via a protective insulating film in a plan view. The liquid crystal panel of the FFS mode has an advantage of having a higher aperture ratio than that in the liquid crystal panel of the IPS mode.
In the liquid crystal panel of the FFS mode, a common electrode signal to be applied to the common electrode is externally inputted. For applying the inputted common electrode signal to the common electrode, a common main wiring is formed in an outer peripheral portion (referred to as a picture-frame region) of a display region of the active matrix substrate. The common main wiring preferably has a portion (hereinafter referred to as a first common main wiring) formed in the same wiring layer as the gate line, and a portion (hereinafter referred to as a second common main wiring) formed in the same wiring layer as the data line. With the use of two kinds of common main wirings, by properly selecting the wiring layer of the common main wiring at an intersection of the gate line or the data line and the common main wiring, it is possible to reduce the number of times connecting the gate line and the number of times connecting the data line, and reduce the resistance of the gate line and the data line. Further, by selecting the wiring layer with small resistance and forming the common main wiring in the selected wiring layer, the resistance of the common main wiring can be reduced.
In the active matrix substrate having the two kinds of common main wirings, a connecting circuit is needed for electrically connecting the common main wirings formed in different wiring layers. For example, there is used a connecting circuit for electrically connecting the two kinds of common main wirings by using a connecting electrode formed integrally with the common electrode. Patent Document 1 describes an example of the connecting circuit for electrically connecting the common electrode and the two kinds of common main wirings in the liquid crystal panel of the FFS mode.
An active matrix substrate for the liquid crystal panel of the FFS mode is manufactured using five or six photomasks. Patent Document 2 describes a method for manufacturing the active matrix substrate using five photomasks. In the manufacturing method described in Patent Document 2, a semiconductor layer is patterned using a photomask for a source layer, without using a photomask for a semiconductor layer, and a channel region of a TFT is formed using a photomask for a pixel electrode layer.